1. Field of the Invention
The present invention relates to printed circuit boards, and in particular, to a printed circuit board with a differential signal layout on different layers.
2. Description of the Prior Art
According to the Universal Serial Bus (USB) 2.0 specification, differential mode impedance and common mode impedance of a high speed differential signal are required to be 90 Ohms (±15%) and 30 Ohms (±30%) respectively. However, in general embodiments of edge coupling on a microstrip, a signal layer needs to be very close to a ground layer to lower impedance, and a trace width of a signal trace also varies with a space between the signal layer and the ground layer.
For example, a total thickness of a four-layer Printed Circuit Board (PCB) is approximately 62 mil in general, where 40 mil is 1 mm. A first layer is the signal layer and a second layer is the ground layer with a distance between the two layers being 4.5 mil. If the differential signal trace width and spaces between the differential signal traces are kept within appropriate distances, for instance the width is about 7 mil and the space is about 8 mil, the common mode and differential mode impedances meet the specification mentioned. However, this is difficult to apply to a typical 2-layer PCB.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a side view of a four-layer PCB 1 in the prior art, and FIG. 2 is a diagram of a signal layer 12 from the four-layer PCB 1 in FIG. 1. The PCB 1 comprises three insulation layers 11, 13, and 15, two signal layers 12 and 16, a ground layer 10, and a power layer 14. A thickness of the insulation layer 13 is larger than each thickness of the insulation layers 11 and 15. The purpose of this is that the insulation layer 13 supports the entire PCB 1 while the insulation layers 11 and 15 are used to isolate different signal layers to avoid short-circuiting. The signal layer 12 and the ground layer 10 are 4.5 mil apart, which is the thickness of the insulation layer 15.
There are a positive differential signal trace 22 and a negative differential signal trace 24 that both are disposed on the signal layer 12, and a space 20 between the two traces. Since the thickness of the insulation layer 15 is approximately 4.5 mil, the positive differential signal trace 22 and the negative differential signal trace 24 must each have widths of about 7 mil to meet the USB 2.0 specification, and therefore the space 20 will be about 8 mil. To realize this on a two layer PCB, as the signal layer 12 and the ground layer 10 need to be 4.5 mil apart, the PCB can be easily bent and it is also problematic in industrial technologies as well as not being cost effective.